The invention relates to coherent demodulation of a digitally modulated carrier by phase shift or by frequency shift with continuous phase, which carrier is transmitted as bursts regularly spaced over time, with a continuity of the phase of one burst to the following one. The digital modulation by phase shift may be anyone of the types in current use: it may be especially be PSK 2-4 modulation, in which the carrier is modulated by phase jumps of 90.degree..
The invention finds a particularly important use in the field of radiobroadcasting by satellite using a signal which consists of time-division between an analog carrier 11 frequency modulated by the image signal during the active duration of a television line and a digital carrier 10 emitted during the blanking periods. Referring to FIG. 1, there is shown a map of this multiplex which is used in the C-MAC system as described in "Multiple sound channels in satellite broadcasting" by M. d. Windrawn, I.E.E. Proc. Vol. 129, Pt. A. No. 7, September 1982, pp. 528-531. This system enables eight high quality digital sound tracks associated with the television image to be transmitted. The characteristics of the standard system are as follows:
______________________________________ Image modulation: Frequency modulation ______________________________________ Modulation of the digital carrier: PSK 2-4 Instantaneous flow rate of the digital 20.25 Mbits/s carrier: Duration of a burst: 9.97 .mu. sec. ______________________________________
In the case where synchronization is transmitted during the TV frame, the entire duration of the burst may be used for sending useful data. It is not necessary to transmit a preamble at the beginning of each burst as is conventionally done in systems with time division multiple access (TDMA).
Methods of demodulation applicable to C-MAC satellite communications are already known, particularly using the PSK 2-4 modulation at present proposed.
Among these methods, differential demodulation is at present proposed, because it is simpler to use than coherent demodulation. On the other hand, differential demodulation presents performances as regards noise inferior to those of coherent demodulation. In particular, in satellite broadcasting, differential demodulation does not permit to guarantee simultaneous recovery of the image and sound as soon as the carrier to noise ratio is less than a value which is higher than with coherent demodulation. It has been found experimentally that the interruption of service for the sound takes place for a carrier to noise ratio equal to about 7.0 dB with differential demodulation and equal to 5.5 dB for coherent demodulation, in a 27 MHz wide channel.
It is an object of the invention to propose to provide a method of coherent demodulation (of the type according to which the carrier frequency is recovered by squaring the carrier and selecting the carrier frequency in the resulting spectrum, which has a greater immunity to noise than the prior art methods of differential demodulation and offers increased operating reliability as regards carrier recovery.
Before describing the invention, it may be useful to recall the operating principle of coherent demodulators.
If it is assumed that the modulated carrier 10 is transmitted as bursts of duration T.sub.N with a repeat period T.sub.L, the carrier can be written: EQU x(t)=u(t).s(t)
where u(t) is a periodic signal whose shape is as shown in FIG. 2, of period T.sub.L, such that: EQU u(t)=1 for t.epsilon.[O, T.sub.N ] EQU u(t)=0 for t .cndot.[O, T.sub.N ].
In the case of PSK-2demodulation (also known as MDP-2) with two phase states, s(t) may be written: EQU s(t)=A.SIGMA.a.sub.K r(t-kT).multidot.cos (2.pi.f.sub.o t+.psi..sub.o)
with:
a.sub.k =.+-.1 according to the transmitted bit PA0 A: amplitude of the carrier PA0 f.sub.o : frequency of the carrier PA0 .psi..sub.o : origin phase PA0 r(t): wave shape of the symbol transmitted
T: duration of a bit.
In the case of the modulation of PSK 2-4 or MSK type, s(t) has the general form: EQU s(t)=A.SIGMA.a.sub.k r(t-KT).multidot.cos (2.pi.f.sub.o T+.psi..sub.o -K.pi./2).
Coherent demodulation can be carried out by multiplication of the modulated signal and the recovered carrier, whose frequency is f.sub.o in the case of an PSK-2 modulation and (f.sub.o -1/4T) in the case of an PSK 2-4 or MSK (minimum shift keying) modulation.
This demodulation may be effected by a circuit of the type shown in FIG. 3 receiving the modulated carrier and comprising a band-pass filter 12 whose output is connected to a multiplier 14 and to a carrier recovery circuit 16. The output of the circuit 16 is applied to the second input of the multiplier and the output of the latter is applied to a band-pass filter 18 whose output S provides the demodulated signal. Circuit 16 uses conventionally the mode of recovery of the carrier of a two states modulated signal by squaring this carrier (for example by means of an analog multiplier 18), extracting the component of the output signal at a frequency close to a value 2f.sub.1 which is a function of f.sub.o with the filter and, finally, dividing by 2 in a divider 22.
The signal obtained at the output of the multiplier 18 contains a component, around frequency 2f.sub.1, which may be written: EQU y(t)=Bu(t).multidot.cos (2.omega..sub.1 t+2.psi..sub.o)
with: ##EQU1##
The transmission signal of the bursts u(t) can be decomposed into Fourier series and written: ##EQU2##
As a result, signal y(t) consists of a sum of sine shaped signals of frequencies 2f.sub.1 .+-.n/T.sub.L ##EQU3##
However it is observed that, in the usual case where T.sub.N is very much lesser than T.sub.L, there are several signals which have amplitudes of the same order of magnitude.
For example, with T.sub.N =1 .mu.sec and T.sub.L =64 .mu.sec:
______________________________________ .sup. n 0 1 2 3 4 ______________________________________ .alpha..sub.n 0.156 0.150 0.132 0.105 0.074 ______________________________________
In the embodiment of FIG. 3, the component at frequency 2f.sub.1 is selected with a filter which must have a narrow band whose width is less than 1/T.sub.L. Such a band width may be obtained with a crystal driven filter. However, in general, and particularly in satellite broadcasting, the frequency of the modulated carrier applied to the input of the recovery circuit 16 has an accuracy less than 1/T.sub.L. This may result into selection of a frequency line f.sub.o .+-.n/T.sub.L, with n.noteq.0, which results in considerable phase error on demodulation.
The invention overcomes the drawback. In the method according to the invention, an acquisition burst having the carrier frequency, is transmitted at intervals separated by a period (T.sub.T) longer by at least one order of magnitude than the period of transmission of successive modulated carrier bursts, and the acquisition burst is subjected to the carrier extraction process so as to identify the useful line in the spectrum.
The amplitude of the output signal of narrow band pass filter 20 (FIG. 3) increases with the duration of the acquisition burst, as consequently does the ratio between the amplitude of the useful line and the amplitude of a possible interferring line. The use of an acquisition burst of duration T.sub.S greater than the duration T.sub.N of each useful burst of frequency f.sub.1 enables a much higher ratio to be obtained between the amplitude of the useful line and the amplitude of an interferring line, due to the fact that the amplitude of the output signal from the narrow band pass filter 20 increases with the duration of the burst.
Transmission of the acquisition burst will be controlled by a periodical signal v(t) of period T.sub.t, having the shape shown in FIG. 4 and which may be written: EQU v(t)=1 for t.epsilon.[O,T.sub.S ] EQU v(t)=0 for t [O,T.sub.S ]
with: EQU T.sub.T &gt;&gt;T.sub.L EQU T.sub.S =T.sub.L -T.sub.N.
When used for satellite broadcasting, the acquisition signal v(t) is transmitted during one line of frame blanking and the durations of the various signals transmitted can then be: EQU T.sub.N =10 .mu.s EQU T.sub.L =64 .mu.s EQU T.sub.S =54 .mu.s EQU T.sub.T =20 ms.
The burst must correspond to transmission of frequency f.sub.1 and may hence be constituted by a sequence of bits at zero level.
The signal around frequency 2f.sub.1 obtained after squaring the modulated carrier by a circuit 18 is: ##EQU4##
The latter formula shows that the component at frequency 2f.sub.1, and it alone, is amplitude modulated by signal v(t). Modulation enables f.sub.1 to be identified and the ambiguity on carrier recovery from subsequent useful bursts to be lifted.
Before explaining particular embodiments which enable this modulation to be used to lift the ambiguity on f.sub.1, it is appropriate to indicate the action on a signal y.sub.1 of a rectangular filter of central frequency f.sub.c, with a narrow band W such as 1/T.sub.L &gt;W&gt;&gt;1/T.sub.T (crystal filter, for example).
The transfer function H(.nu.) of this filter has the expression: ##EQU5##
In the case, which is that desired for the filter 20 in FIG. 3, where one has f.sub.c =2f.sub.1, the signal y.sub.2 (t) at the output of the filter has the expression: ##EQU6##
It is seen that the amplitude of y.sub.2 (t) is maximum for t=K.multidot.T.sub.T and has the value v.sub.max =B [.alpha..sub.o +WT.sub.s ].
If, on the contrary, f.sub.c =2f.sub.1 .+-.n/T.sub.L with n.noteq.0 (i.e. if there was an error in the selection of the line), the amplitude of the signal y.sub.2 (t) has the value V.sub.s =B.alpha..sub.n.
For satellite broadcast according to European standards: ##EQU7## than it will be possible to accept the following values:
______________________________________ Useful burst: T.sub.N = 10 .mu. sec. Acquisition burst: T.sub.s = 54 .mu. sec. Band Width: W = 10 kHz. ______________________________________
Which leads to: EQU .alpha..sub.0 +WT.sub.s =0.696 EQU .alpha..sub.1 =0.150 EQU .alpha..sub.2 =0.132.
It is found that, for all values of t which are multiples of kT.sub.T, the amplitude of the frequency component 2f.sub.1 is higher by 13 dB than that of the other components, at frequencies 2f.sub.1 .+-.n/T.sub.L. It is then easy, even in the presence of noise, to identify this line by an amplitude detection followed by a comparison to a threshold and to acquire the identified line.
In this case, the line at frequency (2f.sub.1) is easily identifiable as that having a maximum amplitude higher than a predetermined threshold.
In a particular embodiment of the invention, the useful bursts and acquisition bursts are squared and are mixed with the output signal of a voltage controlled variable frequency oscillator after it has also been squared, and the control voltage of the oscillator is modified progressively until the amplitude of the signal derived from mixing and subjected to narrow band pass filtering exceeds a predetermined threshold. The frequency modification of the oscillator can be done in steps substantially equal to the filtering band width.
It is also an object of the invention to provide a device for coherent demodulation of a carrier digitally modulated by phase shift or frequency shift with phase continuity, which carrier is transmitted as useful bursts spaced regularly in time, with phase continuity from one burst to the next. In an advantageous embodiment, the device comprises means for squaring the modulated frequency (f.sub.1) carrier to cause the appearance of frequency (2f.sub.1), a narrow band pass frequency isolating filter and a divider of frequency by two. The device further comprises means for shifting, before application to the narrow band pass filter, the double frequency (2f.sub.1) of the carrier frequency by an adjustable amount (2f.sub.1 -f.sub.c), means for comparing the amplitude of the output signal of the filter with a threshold and modifying the frequency shift until the amplitude exceeds the threshold and then locking the frequency shift.
The invention will be better understood from the following description of particular embodiments given by way of examples.